Shout more warnings when requested JDK is not supported

Related to travis-ci/travis-ci#1291 and travis-cookbooks#236.

Note: `warn_sunjdk6_eol` is extra sugar, only `warn_jdk_not_found` matters. (By the way, I now realize that `warn_jdk_not_supported` may sound better...)

Possible future improvements: dynamically list the java alternatives and auto-support them if not listed in case list.